Image processing method for increasing the blurring of pirated images

ABSTRACT

The image is broken down into a breakdown color space. For each broken down source pixel, if a source point P S0j  is used to represent the end of the color source vector OP S0j  associated with said source pixel, component points P 1j , P 2j , . . . , P kj , . . . , P nj  are used to represent the ends of the color component vectors OP 1j , OP 2j , . . . , OP kj , . . . , OP nj  associated with the component pixels, if a limit sphere of radius L S0j  is defined, centered on the point P S0j , then said breakdown is such that the following applies: P S0 P j1 , P S0 P j2 , P S0 P jk , . . . , P S0 P jn ≧K S0j ×L S0j , with K S0j ≧0.5. The effectiveness of the scrambling is then significantly enhanced.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/EP2006/069921, filed Dec. 19, 2006, whichwas published in accordance with PCT Article 21(2) on Jun. 28, 2007 inFrench and which claims the benefit of French patent application No.0553964, filed Dec. 20, 2005.

BACKGROUND

1. Technical Field

The invention relates to a method of processing a source image (I_(S))comprising a step for decomposing this image (I_(S)) into a series of“component” images (I_(C1), I_(C2), . . . , I_(Ck), . . . , I_(Cn))which are different, said decomposition being adapted so that thesuccessive viewing of the images of this series at a frequency greaterthan the color fusion frequency for the human eye produces a fused image(I_(F)) which is identical or quasi-identical to said source image.

2. Description of the Related Art

Such an image processing method is described in the documentWO05/027529—THOMSON.

Generally, the difference between the “component” images relates to aplurality of pixels that is large enough to be more than perceptible tothe eye. Thus, in cases of illegal photographing, for example using acamcorder that is not synchronized with the duly processed sourceimages, a difference appears at the level of this plurality of pixelswhen illegally stored images are viewed, which degrades and considerablyscrambles the viewing.

A match between the source image and the fused image means that anydifferences between these two images are not perceptible to the eye, soas not to degrade the direct, and therefore legal, viewing of theimages.

Generally, to display an image using an image display device comprisinga display screen provided with a two-dimensional matrix of elementarypolychrome displays, and means for controlling each elementary displayaccording to a video data triplet, there is associated with each pixelof this image, on the one hand, a display of the screen, on the otherhand a video data triplet (D_(R), D_(G), D_(B)) and each display of thescreen with which a pixel of the image is associated is controlled usingthe video data triplet (D_(R), D_(G), D_(B)) associated with this pixel.By convention, each video data triplet associated with a pixel of theimage to be displayed forms, in a color space linked to the displaydevice, the coordinates of what is called the color vector of thispixel. By definition, the set of the possible values of the video dataor color vector triplets describes, in this color space associated withthe device, a three-dimensional color gamut.

In the case of the display of television sequences, the video data isgenerally standardized, for example in the PAL system or in the NTSCsystem, which are both so-called luminance-chrominance systems. Todisplay an image, a PAL image display device therefore receives, in theform of electrical signals, the video data triplets, generally denoted(Y, U, V), which correspond to the set of the color vectors andtherefore of the pixels of this image, in the PAL color space linked tothis device. Similarly, to display an image, an NTSC image displaydevice therefore receives, in the form of electrical signals, the videodata triplets, generally denoted (Y, I, Q), which correspond to the setof the color vectors and therefore of the pixels of this image, in theNTSC color space linked to this device. By extension, the terms YUV andYIQ spaces are used, in which Y designates the luminance, and in which Uand V, or I and Q, designate the chrominance.

The video data is generally “gammatized” (set to the power of a “gamma”factor) to take account of the voltage response of cathode ray tubedisplays.

When it is applied to the display of an image sequence using an imagedisplay device, the image processing method described in theabovementioned document WO05/027529 results in an image display method.

As indicated in this document, such a display method then makes itpossible:

-   -   on the one hand, to display any sequence of processed source        images so that the observer perceives the source images of this        sequence as if none of it were processed;    -   on the other hand, to scramble the photographing of the display        of this sequence of processed images, that might be attempted by        an ill-intentioned person, notably using a camcorder that is not        synchronized with the sequence of source images.

In this document, the decomposition of the source images is performed ina color space denoted YUV, that is a PAL color space; as indicatedhereinabove, this space depends on the image display device used todisplay the images of the sequence. To decompose a pixel of the sourceimage I₁, to which corresponds a color vector of coordinates (Y₁, U₁,V₁) in this color space, this “source” vector is decomposed into two“component” vectors respectively of coordinates (Y₃, U₃, V₃), (Y₄, U₄,V₄), in which Y₁=Y₃=Y₄; again according to this document, the values ofU₃, V₃, U₄, V₄ are determined so that the ends C₃ and C₄ of these two“component” vectors:

-   -   have a barycenter which corresponds to the end C₁ of the color        source vector, that is, are symmetrical relative to this end C₁        in the same PAL color space linked to the display device;    -   are located in the color gamut of this device.

Thus, the essential criteria for the decomposition of a color vector ofa pixel of the source image into two color vectors, each associated witha pixel of a component image are that, in the color space YUV, forexample, the points C₁, C₃ and C₄ are aligned and that the Euclidiandistance C₁C₃ is equal to the Euclidian distance C₁C₄.

It has been observed that the decomposition of source images asdescribed in the document WO05/027529 did not always result in aneffective scrambling of the images after illegal recording, for exampleusing a camcorder. One aim of the invention is to enhance the scramblingof the processed images.

SUMMARY

To this end, there is notably proposed a means for increasing thedifferences between the composite images by a refinement of thedecomposition of the source images.

To this end, the subject of the invention is a method of processing asource image I_(S0), in which said source image I_(S0) is decomposed ina decomposition color space into a series of n “component” imagesI_(C1), I_(C2), . . . , I_(Ck), . . . , I_(Cn) and in which each sourcepixel E_(S0j) which belongs to a plurality of decomposed pixels E_(S01),E_(S02), . . . , E_(S0j), . . . , E_(S0q) of this source image I_(S0)has a corresponding series of component pixels E_(C1j), E_(C2j), . . . ,E_(Ckj), . . . , E_(Cnj) respectively in each of said component imagesI_(C1), I_(C2), . . . , I_(Ck), . . . , I_(Cn),

wherein,

-   -   if, in said decomposition space, the set of the color vectors        accessible to a display device intended to display said source        image forms a three-dimensional gamut of colors,    -   and if, for each source pixel E_(S0j) which belongs to said        plurality of decomposed pixels E_(S01), E_(S02), . . . ,        E_(S0j), . . . , E_(S0q), a source point P_(S0j) is used to        represent the end of the color source vector OP_(S0j) associated        with said source pixel E_(S0j), and component points P_(1j),        P_(2j), . . . , P_(kj), . . . , P_(nj) are used to represent the        ends of the color component vectors OP_(1j), OP_(2j), . . . ,        OP_(kj), . . . , OP_(nj) associated with said component pixels        E_(C1j), E_(C2j), . . . , E_(Ckj), . . . , E_(Cnj) of the series        which corresponds to said source pixel (E_(S0j)), said        decomposition is such that, for each source pixel (E_(S0j)) of        said plurality, said component points P_(1j), P_(2j), . . . ,        P_(kj), . . . , P_(nj) associated with the component pixels        (E_(C1j), E_(C2j), . . . , E_(Ckj), . . . , E_(Cnj))        corresponding to said source pixel (E_(S0j)) are all located in        said three-dimensional color gamut and the barycenter of said        component points P_(1j), P_(2j), . . . , P_(kj), . . . , P_(nj)        corresponds approximately to said source point P_(S0j),        and, also for each source pixel (E_(S0j)) of said plurality,    -   if, still within said decomposition space, there is defined a        limit sphere which is centered on the source point P_(S0j)        associated with said source pixel (E_(S0j)) and the surface of        which contains a series of n “limit” points P_(1jL), P_(2jL), .        . . , P_(kjL), . . . , P_(njL), the barycenter of which        approximately coincides with the source point P_(S0j) and which        present the greatest possible distance between them while being        included, including limits, in said three-dimensional gamut,    -   and, if a scrambling factor K_(S0j) of said source pixel E_(S0j)        of said source image I_(S0) is defined which is less than or        equal to 1 and greater than or equal to 0.5,        said decomposition is also such that the distances between said        source point P_(S0j) and each of said component points P_(1j),        P_(2j), . . . , P_(kj), . . . , P_(nj) associated with the        component pixels (E_(C1j), E_(C2j), . . . , E_(Ckj), . . . ,        E_(Cnj)) corresponding to said source pixel (E_(S0j)) are all        greater than or equal to K_(S0j) times the radius L_(S0j) of        said limit sphere.

The source image comprises q pixels which are decomposed, the otherpixels of this source image not then being decomposed; the pixels thatare not decomposed remain as such unchanged in each of the componentimages; conversely, each decomposed pixel has a corresponding series ofn component pixels, at least two of the pixels of this series beingdifferent, so that the n “component” images I_(C1), I_(C2), . . . ,I_(Ck), . . . , I_(Cn) are effectively differentiated; the set of thedecomposed pixels E_(S01), E_(S02), . . . , E_(S0j), . . . , E_(S0q) ofthe source image form a plurality of q decomposed pixels.

Each pixel of the image to be decomposed is generally associated with avideo data triplet (D_(R), D_(G), D_(B)) which is able to control anelementary display of a display device so as to obtain the display ofsaid pixel, and which by convention forms the coordinates of the colorvector associated with said pixel of this image in a color spaceassociated with said device; the set of the possible values of the videodata triplets then describes, in this color space associated with saiddevice, a three-dimensional gamut of colors, which can be transposedinto the decomposition color space, at least in the case where thedecomposition color space is different from the color space linked tothe device.

K_(S0j) is therefore the scrambling factor of a pixel E_(S0j) of thesource image I_(S0). The following then apply:P_(S0)P_(j1)≧K_(S0j)×L_(S0j); P_(S0)P_(j2)≧K_(S0j)×L_(S0j), . . . ,P_(S0)P_(jk)≧K_(S0j)×L_(S0j), . . . , P_(S0)P_(jn)≧×K_(S0j)×L_(S0j). Itshould be noted that this scrambling factor is differentiated from themetamerization parameter t₄ defined in the document US2004/081318 whichproposes another solution for enhancing the scrambling: instead of asequential decomposition of source images as in the invention, thisdocument proposes to “encode”, without decomposing them, the sourceimages into at least four primary colors (instead of three), so as tohave a freedom parameter (t₄) that can be used to scramble a camcorder(see §30); this document does not define any scrambling optimizationcriterion which is related to the position of the ends of color vectorsin a sphere of the color space, as in the invention; it should be notedthat the two methods can be used simultaneously without departing fromthe invention.

Said display device used to display the decomposed images can be avirtual device; for example, if the color space linked to the device isof YUV type, the associated virtual device is a PAL television.

In the decomposition color space, each series of n “component” images(I_(C1), I_(C2), . . . , I_(Ck), . . . , I_(Cn)) is differentiated by aplurality of component pixels of which the resultant of the colorvectors is equal or quasi-equal to n times the color vectors associatedwith the decomposed pixels (I_(S01), E_(S02), . . . , E_(S0j), . . . ,E_(S0q)) of said source image (I_(S0)). The resultant of the componentcolor vectors OP_(1j), OP_(2j), . . . , OP_(kj), . . . , OP_(nj)associated with the component pixels (E_(C1j), E_(C2j), . . . , E_(Ckj),. . . , E_(Cnj)) of each component image (I_(C1), I_(C2j), . . . ,I_(Ck), . . . , I_(Cn)) is therefore equal or quasi-equal to n times thecolor source vector OP_(S0j) associated with the decomposed source pixelE_(S0j) of the source image I_(S0), in as much as the fusion of thecomponent pixels gives, approximately for the eye, the source pixel; theterm “fusion” should be understood to mean the perception of the eye onthe successive display of each component image at a frequency greaterthan that of the fusion of the colors to the eye. The ends P_(S0j),P_(1j), P_(2j), . . . , P_(kj), . . . , P_(nj) of the color vectors arethen arranged so that the barycenter of the component points P_(1j),P_(2j), . . . , P_(kj), . . . , P_(nj) corresponds to the source pointP_(S0j); preferably, the luminance values corresponding to the set ofthese color vectors are identical (Y_(S0j)=Y_(1j)=Y_(2j)= . . . =Y_(kj)=. . . =Y_(nj) then applies).

Only a plurality of pixels of the source image are decomposed, the otherso-called “unchanging” pixels being carried as such into each of thecomponent images; in a source image, this plurality of decomposed pixelscan form a pattern, such as, for example, a message stating thecopyrights attached to the image. The position and the size of thepattern that appears in the component images can advantageously beadapted to optimize the perception of them by the eye, so as to furtherincrease the scrambling. According to a variant, most of the pixels ofthe source image are decomposed, and it is the “unchanging” pixels whichform a pattern that is inscribed, as it were, in “negative” on theimage.

Said decomposition space can be a YUV (PAL) or YIQ (NTSC) space, thesebeing spaces linked to a display device; it can also be a space XYZ,Yxy, or linearly derived from these spaces, like the Ycd space describedhereinbelow, which are luminance-chrominance spaces independent of thedisplay device; other color spaces can be used for the decompositionwithout departing from the invention.

Preferably, said decomposition space is a perceptually uniform colorspace. Thanks to the choice of this decomposition space, not only doesthe source image processing according to the invention make it possibleto guarantee a greater difference than in the prior art between thecomponent images of this source image, but also this difference is nowoptimized from the point of view of perception by the human eye, whichmakes it possible to further enhance the scrambling. For theperceptually uniform decomposition space, it is possible to choose thespace CIE-LAB, the space CIE-LUV, or the space QMH, or even the spaceJCH.

When applied to the display of a sequence of source images at a givensource frequency using a display device, the image processing methodaccording to the invention makes it possible to guarantee, for eachdecomposed source image, a greater difference than in the prior artbetween the component images of this source image, which makes itpossible to optimize the scrambling of the images. The component imagesof each series are then displayed at a frequency equal to n times thesource frequency, which is greater than the color fusion frequency forthe human eye. For each given decomposed source image I_(S0), a minimumdifference, linked to the factors K_(S0j), is assured. Preferably, thefactor K_(S0j) is common to all the decomposed pixels of the sourceimage I_(S0). Moreover, this difference is optimized in relation to thedisplay capabilities of the device, since the color vectors associatedwith the pixels of the component images remain within, limits included,the three-dimensional color gamut of this device.

According to first variant, for said decomposition, the decompositionspace comprises the absolute luminance quantity and, one of thecoordinates of said source point P_(S0j) then corresponding to aluminance Y_(S0j), then, for said decomposition, the series of the n“limit” points P_(1jL), P_(2jL), . . . , P_(kjL), . . . , P_(njL), theseries of the n component points P_(1j), P_(2j), . . . , P_(kj), . . . ,P_(nj) are chosen in one and the same constant luminance planecorresponding to the luminance coordinate Y_(S0j) of said source point.Obviously, said source point P_(S0j) also belongs to this constantluminance plane.

If a two-dimensional gamut is then defined by the intersection of theconstant luminance plane Y_(S0j) with said three-dimensional gamut stilltransposed into said decomposition color space, then the n “limit”points P_(1jL), P_(2jL), . . . , P_(kjL), . . . , P_(njL) are positionedon a circle of radius L_(S0j) which is included, including limit, insaid two-dimensional gamut.

According to a second variant, for said decomposition, the series of then component points P_(1j), P_(2j), . . . , P_(kj), . . . , P_(nj) arechosen so as not to belong to any plane perpendicular to one of thereference axes of said decomposition space. In particular, if thedecomposition space comprises the absolute luminance quantity, thedecomposition then leads to a luminance modulation.

Preferably, this second variant is used notably for processing asequence of source images in which each of the source images isprocessed according to the invention, and the decompositions of saidsource images are carried out so that the succession of the series ofcomponent pixels corresponding to the decomposed pixels of these sourceimages result in a luminance fluctuation at a frequency less than theflicker limit frequency of the human eye, preferably less than or equalto 20 Hz. Preferably, said source space comprises the luminancequantity. For example, in the case of two successive source imagesI_(S0-a), I_(S0-b) of a sequence that are each decomposed into twocomponent images I_(C1-a), I_(C2-a) and I_(C1-b), I_(C2-b), eachdecomposed pixel E_(S0j-a) of the first source image I_(S0a) will bedecomposed into two component pixels, E_(C1j-a), E_(C2j-a), so that theluminance of the first component pixel E_(C1j-a) is greater than that ofthe second component pixel E_(C2j-a); each decomposed pixel E_(S0j-b) ofthe second source image I_(S0-b) will be decomposed into two componentpixels E_(C1j-b), E_(C2j-b) so that the luminance of the first componentpixel E_(C1j-b) is in this case, conversely, less than that of thesecond component pixel E_(C2j-b); then, the succession of two sourcepixels E_(S0j-a)-E_(S0j-b), which is reflected by the followingsuccession of the component pixels:E_(C1j-a)-E_(C2j-a)-E_(C1j-b)-E_(C2j-b), will produce a luminancefluctuation which, when it is convoluted with the frequency of theshutter of a camcorder, will generate a flicker, making the illegalphotograph unusable. The scrambling of the images is therefore furtherenhanced, but it is important here to be careful in particular not todegrade the display of the images by avoiding this flicker effect when“legally” displaying the images.

Preferably, whether for the first or the second variant, said scramblingfactor K_(S0)=K_(S0j) is common to said plurality of decomposed pixels(I_(S01), E_(S02), . . . , E_(S0j), . . . , E_(S0q)) of said sourceimage I_(S0j). The following then applies: K_(S01)=K_(S02)= . . .K_(S0j)= . . . =K_(S0q)=K_(S0).

Preferably, said scrambling factor K_(S0j) is greater than or equal to0.8. The differences between the component images of the source imagesis further enhanced and the scrambling of the images is furtherincreased.

Preferably, said component points P_(1jL), P_(2j), . . . , P_(kj), . . ., P_(nj) forming the edges of a polyhedron, said polyhedron isequilateral and centered on said source point P_(S0). These componentpoints P_(1j), P_(2j), . . . , P_(kj), . . . , P_(nj) are therefore onone and the same sphere, or, as appropriate, one and the same circle,centered on P_(S0), and of radius K_(S0j)×L_(S0j). Thanks to theequilateral nature of the polyhedron, that is, to the equidistance ofthe component points, the differences between the component images ofthe source images are further enhanced and the scrambling of the imagesis further increased.

Another subject of the invention is a method of processing a sequence ofsource images in which at least one of said images is processedaccording to the invention, in which,

if each source pixel (E_(S0j)) of at least one processed source imageI_(S0) which belongs to said plurality of decomposed pixels (E_(S01),E_(S02), . . . , E_(S0j), . . . , E_(S0q)) has associated with it amotion vector and there is defined an upper scrambling limit M_(S0j) ofsaid source pixel E_(S0j) which is greater than the scrambling factorK_(S0j) associated with said source pixel and which is such that thedistances between the end P_(S0j) of the color source vector associatedwith said source pixel (E_(S0j)) and each of the ends P_(1j), P_(2j), .. . , P_(kj), . . . , P_(nj) of the color component vectors associatedwith the component pixels (E_(C1j), E_(C2j), . . . , E_(Ckj), . . . ,E_(Cnj)) corresponding to said source pixel (E_(S0j)) are all less thanor equal to M_(S0j) times the radius L_(S0j) of the limit sphereassociated with said source pixel (E_(S0j)),then, for each (E_(S0j)) of said source pixels of said plurality, saidupper scrambling limit M_(S0j) is inversely proportional to the modulusof the motion vector of said source pixel (E_(S0j)).

According to the invention, the difference between the component imagesof a given source image is therefore reduced in the areas of this sourceimage which are affected by a significant motion in the course of thevideo sequence of images to be displayed. When the sequence of sourceimages is derived from a decompression operation using standardizedprotocols, for example of MPEG type, the motion vector data isimmediately available for each pixel of these images; this motion vectorcan be common to the various pixels of one and the same decompressionmacro block.

Preferably, so as to obtain a motion-dependent sliding “scramblingband”, still for each E_(S0j) of said source pixels of the plurality ofdecomposed pixels, the scrambling factor K_(S0j) associated with saidsource pixel is inversely proportional to the modulus of the motionvector of this source pixel (E_(S0j)).

Preferably, the scrambling factor is common to said plurality ofdecomposed pixels of each source image I_(S0j). According to a variant,the scrambling factor is also common to all the source images.

Another subject of the invention is a method of displaying a sequence ofimages intended for a given source frequency, comprising at least oneseries of component images (I_(C1), I_(C2), . . . , I_(Ck), . . . ,I_(Cn)) obtained by the processing of at least one source image I_(S0)according to the invention, or by the processing of a sequence of imagesaccording to the invention, in which each (I_(Ck)) of said “component”images is successively displayed at a component frequency which is equalto n times said source frequency and which is greater than the colorfusion frequency of the human eye.

With reference to FIG. 3, another subject of the invention is a device300 for displaying a source image, each pixel of which is associatedwith a video data triplet (D_(R), D_(G), D_(B)), comprising:

-   -   a display panel 302 comprising a two-dimensional matrix of        elementary polychrome displays;    -   control means 304 able to control each elementary display using        a video data triplet (D_(R), D_(G), D_(B)) associated with a        pixel so as to obtain the display of this pixel;    -   means 306 able to process the source image to be displayed        according to the invention, so as to generate a series of        component images of said source image;    -   in which said control means are adapted to successively display        each component image at a frequency greater than the color        fusion frequency.

An exemplary elementary display of a display screen would be a group ofthree liquid-crystal or micro-mirror valves modulating in threedifferent primary colors, or a group of three light-emitting diodesemitting in three different primary colors. In the case of a projectiondisplay device in which the light of a source is modulated by a liquidcrystal or micro-mirror valve micro-imager, each elementary display isformed by a value.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be better understood from reading the descriptionthat follows, given by way of nonlimiting example, and with reference tothe appended figures in which:

FIG. 1 represents the intersection of different constant planes Y=withthe three-dimensional color gamut of the display device to which theinvention is applied, a gamut that is represented in a color space(Y,c,d) independent of this device which is used in an embodiment of theinvention;

FIG. 2 represents a step for the decomposition of a color vector of asource image into two component image color vectors, in the same colorspace (Y,c,d) independent of this device, according to the sameembodiment of the invention as that of FIG. 1;

FIG. 3 represents a device for displaying a source image;

FIG. 4 represents a source image and component images; and

FIG. 5 represents a two-dimensional frame of reference of chrominancecomponents that illustrates a color source vector.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

There now follows a description of an embodiment of the processingmethod according to the invention applied to the display of a sequenceof images by using an image display device provided with a screencomprising a matrix of display elements and provided with means ofcontrolling these display elements, in which, to obtain the display of agiven image, each pixel of this image has associated with it a videodata triplet (D_(R), D_(G), D_(B)) which, when it is addressed to thedisplay element that corresponds to this pixel, via the control means ofthis device, generates the display of this pixel. Each image of thesequence is partitioned into a pixel matrix, so that each displayelement corresponds to a pixel of this matrix.

Such a display device can immaterially be a digital video projector, anoverhead projector, a plasma screen, an LCD screen or another imagedisplay screen which can be addressed by video data.

With reference to FIG. 4, in the sequence of images to be displayed, asource image 402 is selected to be decomposed I_(S0), in this case intoa series of two component images I_(C1) 404, I_(C2) 406. Certain pixelsof the two component images I_(C1), I_(C2) are identical to those of thesource image I_(S0), others are differentiated from the source image andform a plurality of differentiating pixels: E_(C11), E_(C12), E_(C1j), .. . , E_(C1q) for the component image I_(C1), and E_(C21), E_(C22), . .. , E_(C2j), . . . , E_(C2q) for the component image I_(C2). In eachcomponent image I_(C1), I_(C2) there are therefore q pixels beingdifferentiated from the source image I_(S0), the other pixels beingidentical. The number q of differentiating pixels preferably representsat least 10% of the total number of pixels of an image, so that thedifference between the component images can be perceptible to the eye.The decomposition of the source image I_(S0) which will be describedhereinbelow aims for the fusion of the pixels E_(C11) and E_(C21),E_(C12) and E_(C22), . . . , E_(C1j) and E_(C2j), . . . , E_(C1q) andE_(C2q), of identical positions on all the component images I_(C1),I_(C2) to generate for the human eye a pixel that is identical to that(E_(S01), E_(S02), E_(S0j), . . . , E_(S0q)) of the same position on thesource image l_(C1). By extension, it is therefore said that the sourcepixels (E_(S01), E_(S02), . . . , E_(S0j), . . . , E_(S0q)) aredecomposed into component pixels (E_(C11), E_(C12), . . . , E_(C1j), . .. , E_(C1q)) for the component image I_(C1), and (E_(C21), E_(C22), . .. , E_(C21), . . . , E_(C2q)) for the component image I_(C2).

The pixels of the source image I_(S0) which are decomposed thereforeform the following plurality: E_(S01), E_(S02), . . . , E_(S0j), . . . ,E_(S0q). There now follows a detailed explanation of how to decomposeone of these pixels, E_(S0j), into pixels of the same position E_(C1j)and E_(C2j) respectively of the component images I_(C1), I_(C2), thedecomposition of the other pixels of this plurality being done in thesame way.

This pixel E_(S0j) 408 of the source image I_(S0) 402 has associatedwith it, as seen previously, a video data triplet (D_(R-S0j), D_(G-S0j),D_(B-S0j)); the video data triplets (D_(R-C1j), D_(G-C1j), D_(B-C1j))(D_(R-C2j), D_(G-C2j), D_(B-C2j)) that are respectively associated withthe pixels E_(C1j) 410 and E_(C2j) 412 of the component images I_(C1)404, I_(C2) 406, and which, when they are displayed by the displaydevice at a frequency greater than the color fusion frequency of thehuman eye, generate a pixel identical to E_(S0j) are sought; morespecifically, the video data triplets (D_(R-C1j), D_(G-C1j), D_(B-C1j)),(D_(R-C2j), D_(G-C2j), D_(B-C2j)) that give rise to the display ofpixels E_(C1j) and E_(C2j) that are as different as possible from eachother are sought, so as to enhance the scrambling of the images.

It is now considered that the triplet (D_(R-S0j), D_(G-S0j), D_(B-S0j))associated with the pixel E_(S0j) represents the coordinates of a vectorOP_(S0j), called “color vector”, in a color space associated with thedisplay device.

It is considered here that the video data as must be addressed to thedisplay device is all encoded on 10 bits; each video data item cantherefore take an integer value between 0 and 1023. The three columns oftable 1 hereinbelow give the coordinates of color reference vectors OO,OR, OG, OB, OC, OM, OY and OW, corresponding, in row order, to black,then to each of the primaries of the device (respectively red, green andblue), then to each of the secondaries of the device (respectively cyan,magenta and yellow), then to the reference white of the device. The endsof these color reference vectors therefore delimit a cube in this colorspace, also called three-dimensional gamut, within which, includinglimits, are contained all the color vectors that are displayable by thedisplay elements of the screen.

TABLE 1 D_(R) D_(G) D_(B) X Y Z Y C D OO 0 0 0 0.00 0.00 0.00 0.00 0.000.00 OR 1023 0 0 19.19 8.99 0.97 8.99 2.13 0.11 OG 0 1023 0 10.27 28.282.92 28.28 0.36 0.10 OB 0 0 1023 10.02 3.95 50.64 3.95 2.54 12.82 OC 01023 1023 20.29 32.23 53.56 32.23 0.63 1.66 OM 1023 0 1023 29.22 12.9451.61 12.94 2.26 3.99 OY 1023 1023 0 29.46 37.27 3.89 37.27 0.79 0.10 OW1023 1023 1023 39.49 41.22 54.53 41.22 0.96 1.32

This video data will now be transposed into a known color space XYZ,which is independent of the device, then into a new color space Ycd,also derived from XYZ and independent of the device. In this new colorspace advantageously used for the implementation of the invention, thecoordinates Y, c, d of each color vector OP are expressed as follows:Y=Y, c=X/Y, d=Z/Y. In this new color space, one of the trichromaticcomponents Y represents the luminance of the pixel and the other twotrichromatic components c, d are independent of the luminance andrepresent the chrominance.

Table 1 gives the correlation between the values of the coordinates ofthe eight color reference vectors OO, OR, OG, OB, OC, OM, OY and OW,when they are expressed in the video data space or color space specificto the device, when they are expressed in the space XYZ, and when theyare expressed in the new color space used here for the decomposition.

The correlation D_(R), D_(G), D_(B)→XYZ is established in a manner knownper se, for example by using known display device calorimetriccharacterization methods, such as those described in the standard IEC61966. The spectral visual functions x(λ), y(λ), z(λ) characteristic ofthe XYZ color systems can also be used. The correlation XYZ→Ycd isestablished as defined previously. Since the ends Q, R, G, B, C, M, Yand W of these eight different color reference vectors are peaks of thethree-dimensional gamut of the device which forms a cube in the videodata space, this table gives the coordinates of the peaks of this samethree-dimensional gamut in the new color space. In the latter twospaces, the three-dimensional gamut forms a polyhedron, the peaks ofwhich are formed by the ends of the eight color reference vectors OO,OR, OG, OB, OC, OM, OY and OW.

FIG. 1 represents, in the two-dimensional reference frame of thechrominance components c and d, different intersections of planes Y(luminance)=constant with this polyhedral three-dimensional gamut, theseplanes and this gamut thus being represented in the new color space thatis independent of the device defined previously: intersections of theplane Y=0 in chain dotted-lines, of the plane Y=10 in dotted lines, ofthe plane Y=15 in dashed lines, and of the plane Y=35 in solid lines.These intersections are limited by two-dimensional polygons. Into thistwo-dimensional reference frame of the chrominance components c and d,is also transferred the projection R′, G′, B′, C′, M′, Y′ and W′ ontothese planes Y=constant of the end points R, G, B, C, M, Y and W of thecolor reference vectors OR, OG, OB, OC, OM, OY and OW of table 1. Thecoordinates of these points R′, G′, B′, C′, M′, Y′ and W′ are given incolumns c and d of table 1.

There now follows a return to the pixel E_(S0j) of the source imageI_(S0) to be decomposed, with which is associated the triplet(D_(R-S0j), D_(G-S0j), D_(B-S0j)) which represents, in the color spaceassociated with the display device, the coordinates of the color vectorOP_(S0j) associated with this pixel. The triplet (Y_(S0j), C_(S0j),d_(S0j)) of the coordinates of this same color source vector OP_(S0j),expressed, this time, in the new color space according to the invention,is therefore sought.

To this end, the method entails linear interpolation from colorreference vectors, on the one hand which frame the color vectorOP_(S0j), on the other hand for which the correlation D_(R), D_(G),D_(B)→XYZ→Ycd has been established, as mentioned previously. Such alinear interpolation method is known per se and will not be describedhere in detail.

There is then obtained the triplet (Y_(S0j), C_(S0j), d_(S0j)) of thecoordinates of the color source vector OP_(S0j) expressed in the newindependent color space of the device.

With reference to FIG. 2 and FIG. 5, the intersection of thethree-dimensional gamut of this device in this new color space with theplane Y=Y_(S0j) is sought; this intersection forms a two-dimensionalgamut 1 of luminance Y=Y_(S0j), and represents the set of the colorsaccessible to the display device for this luminance Y=Y_(S0j). In thistwo-dimensional gamut, the end P_(S0j) 502 of the color vector OP_(S0j)504 is positioned; in the two-dimensional frame of reference of thechrominance components c and d located in this two-dimensional gamut,the coordinates of this point P_(S0j) are therefore C_(S0j), d_(S0j). Anarea is now defined, called symmetrical two-dimensional gamut 2, whichis symmetrical to the two-dimensional gamut relative to the pointP_(S0j), still in the same plane Y=Y_(S0j); there is then defined anarea, called reduced two-dimensional gamut 3, which corresponds to theintersection of the two-dimensional gamut and the symmetricaltwo-dimensional gamut.

The coordinates of the points which each delimit polygons(two-dimensional gamut 1, symmetrical two-dimensional gamut 2, reducedtwo-dimensional gamut 3) can be obtained by linear interpolation of thecoordinates of the points R′, G′, B′, C′, M′, Y′ and W′ which are givenin columns c and d of table 1. These coordinates can be used to obtainthe equations algebraically representing these polygons.

Concentric circles 4, 5, 6 are drawn, centered on P_(S0j), which arecontained in the reduced two-dimensional gamut 3 or which represent aline of intersection with this reduced two-dimensional gamut; morespecifically, these circles are defined as follows:

-   -   the limit circle 4 is centered on P_(S0j) and passes through the        limit points P_(1jL) and P_(2jL), symmetrical with P_(S0j),        which are the furthest apart from each other in the reduced        gamut 3; L_(S0j) denotes the distance        P_(S0j)P_(1jL)=P_(S0j)P_(1jL);    -   the minimum circle 5 is centered on P_(S0j) and has a radius        equal to 0.5×L_(S0j);    -   the mean circle 6 is centered on P_(S0j) and has a radius equal        to 0.8×L_(S0j).

The radius L_(S0j) of the limit circle is deduced from the equationalgebraically representing the reduced two-dimensional gamut 3 and fromthe equation expressing that the point PS0j is the barycenter of thelimit points P_(1jL) and P_(2jL). The factors 0.5 and 0.8 correspond topossible values of a so-called scrambling factor K_(S0j) specific to thepixel E_(S0j); this factor can be common to all the pixels of the sourceimage S₀ to be decomposed; conversely, this factor can be variabledepending on the pixels of the source image S₀ to be decomposed,preferably inversely proportional to the motion vector of this pixel, soas advantageously to reduce the scrambling ratio in the parts of theimage subject to strong movement; it can be common to all the sourceimages to be decomposed, or conversely be variable according to thesource images. A mean image scrambling level (K_(S0j)1=0.8) is, forexample, chosen here, which means that, to decompose the color vectorOP_(S0j) 504 associated with the pixel E_(S0j) into two color vectorsOP_(C1j) 506, OP_(C2j) 508 associated at the pixels E_(C1j) and E_(C2j)respectively of the component image I_(C1) and the component imageI_(C2), two symmetrical points P_(C1j) 510, P_(C2j) 512 are chosen onthe mean circle 6, and the respective coordinates (C_(C1j), d_(C1j)),(C_(C2j), d_(C2j)) of these points are evaluated in the previouslydefined two-dimensional frame of reference c, d. By choosing here pointsP_(C1j), P_(C2j) on a circle of relatively high diameter (0.8) relativeto the maximum, the differences between the composite images aresubstantially increased, which increases the effectiveness of thescrambling.

By continuing the decomposition of the pixel E_(S0j) of the source imageS0 into two pixels E_(C1j) and E_(C2j) respectively of the componentimage I_(C1) and the component image I_(c2), Y_(c1j)=Y_(c2j)=Y_(S0j) isdefined, the triplets (Y_(c1j), c_(c1j), d_(c1j)), (Y_(c2j), c_(c2j),d_(c2j)) which express the coordinates of the two color vectorsOP_(c1j), OP_(c2j) in the new color space according to the invention areobtained. By a reverse transformation of the previously defined linearinterpolation which was used to switch from the expression of thecoordinates of a color vector in the color space linked to the device tothe expression of the coordinates of the same vector in the new colorspace of the invention, the triplets (D_(R-C1j), D_(G-C1j), D_(B-C1j)),(D_(R-C2j), D_(G-C2j), D_(B-C2j)) which express the coordinates of thesame two color vectors OP_(c1j) OP_(c2j) are calculated, this time inthe color space linked to the device.

Thus, when the image display device generates the succession of thecomponent images I_(C1), I_(C2) at a frequency greater than the colorfusion frequency of the human eye, the pixels E_(Cj1) and E_(Cj2) of thecomponent images I_(C1), I_(C2) will be displayed successively from thefollowing video data triplets (D_(R-C1j), D_(G-C1j), D_(B-C1j)),(D_(R-C2j), D_(G-C2j), D_(B-C2j)), which will generate, because of thefusion of the colors, a pixel identical to the pixel E_(S0j) of thesource image E_(S0). Conversely, in the image obtained from an illegalrecording using an unsynchronized camera, the observer will see twoimages I_(C1), I_(C2) appear, which will be all the more distinct astheir component pixels are associated with different color vectors: forexample, as defined previously, the pixels E_(C1j) and E_(C2j)respectively of the image I_(C1) and of the image I_(C2) present anoptimal difference for a scrambling factor K_(S0j)=0.8 qualified as“mean”.

This difference between the pixels of the different component images ofone and the same source image can advantageously be modulated, forexample by increasing the number q of differentiating pixels, forexample by changing the size of the scrambling pattern. According to anadvantageous variant, in each source image to be decomposed, thedifferentiating pixels E_(S01), E_(S02), . . . , E_(S0j), . . . ,E_(S0q) are positioned in the areas of the source images where thehighest distances L_(S01), L_(S02), . . . , L_(S0j), . . . , L_(S0q)(defined previously) can be obtained.

The difference between the pixels of the different component images ofone and the same source image can be reinforced by increasing thescrambling factor K_(S0j), notably in the range of values between 0.8and 1 inclusive. To avoid the display defects in sequences of imageshaving areas assigned a high amplitude motion vector, it is possible, onthe contrary, to be forced to lower this scrambling factor, or even tototally cancel it, in which case the operation to decompose the pixelsE_(S0j) located in these areas assigned a high amplitude motion vectoris squarely eliminated; preferably, this scrambling factor remainsgreater than or equal to 0.5 to maintain a sufficient difference betweenthe component images.

In the description of the present invention, the transformations of theexpression of the coordinates of the color vectors between differentcolor spaces have been achieved by linear interpolation; other knowntransformation types can be used without departing from the invention.

The present invention has been described with reference to adecomposition of source images into two component images; decompositionsinto a higher number of component images can be considered withoutdeparting from the invention; to generalize, a source image can thus bedecomposed into a series of n “component” images: I_(C1), I_(C2), . . ., I_(Ck), . . . , I_(Cn); according to a variant, the number n can varyaccording to the source image to be decomposed; indeed, since the colorfusion frequency depends on the luminosity of the images, it is possibleto envisage a higher number n for low fusion frequencies, andvice-versa.

Other color spaces can be envisaged for decomposition the source imagesoptimally without departing from the invention. Preferably, aperceptually uniform space is chosen. For the perceptually uniformdecomposition space, it is possible to choose the space CIE-LAB (alsocalled Lab), the space CIE-LUV (also called Luv), or the space QMH, oreven the space JCH; for the space QCH, Q designates the brightness, Cthe colorfulness, and H the huequadrature or hueangle; for the spaceJCH, J designates the lightness, C designates the “chroma”, and H aspreviously designates the hue. The choice of such a perceptually uniformspace advantageously makes it possible to even further heighten thedifferences between the component images, this time as they areperceived by the human eye, which further reinforces the scramblinglevel; indeed, the choice of such a color space makes it possible tospecifically maximize the perception differences between the componentimages of one and the same source image.

The example that has just been described can be implemented by using,this time, space transformation formulae adapted to the perceptuallyuniform space, formulae that are within the scope of those skilled inthe art. It will be observed that by performing the decomposition in aspace LAB in the same way as in the space Ycd as described previously,substantially different component images are obtained for one and thesame source image. It can be seen that the use of a perceptually uniformspace for the decomposition makes it possible to enhance the scramblingof the images.

The invention applies to other embodiments of the method of displaying asequence of images, without departing from the scope of the claimshereinafter.

1. A method of processing a source image, in which said source image isdecomposed by a hardware element in a decomposition color space into aseries of n component images and in which each source pixel whichbelongs to a plurality of decomposed pixels of this source image has acorresponding series of component pixels respectively in each of saidcomponent images, wherein, if, in said decomposition space, the set ofthe color vectors accessible to a display device intended to displaysaid source image forms a three-dimensional gamut of colors, and if, foreach source pixel which belongs to said plurality of decomposed pixels,a source point P_(S0j) is used to represent the end of the color sourcevector OP_(S0j) associated with said source pixel E_(S0j), and componentpoints P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj) are used torepresent the ends of the color component vectors OP_(1j), OP_(2j), . .. , OP_(kj), . . . , OP_(nj) associated with said component pixels ofthe series which corresponds to said source pixel, said source image isdecomposed such that, for each source pixel of said plurality, saidcomponent points P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj)associated with the component pixels corresponding to said source pixelare all located in said three-dimensional color gamut and the barycenterof said component points P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj)corresponds to said source point P_(S0j), wherein, for each source pixelof said plurality, if, still within said decomposition space, there isdefined a limit sphere which is centered on the source point P_(S0j)associated with said source pixel and the surface of which contains aseries of n limit points P_(ljL), P_(2jL), . . . , P_(kjL), . . . ,P_(njL), the barycenter of which approximately coincides with the sourcepoint P_(S0j) and which present the greatest possible distance betweenthem while being included, including limits, in said three-dimensionalgamut, and, if a scrambling factor K_(S0j) of said source pixel E_(S0j)of said source image I_(S0j) is defined which is less than or equal to 1and greater than or equal to 0.5, said source image is also decomposedsuch that the distances between said source point P_(S0j) and each ofsaid component points P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj)associated with the component pixels corresponding to said source pixelare all greater than or equal to K_(S0j) times the radius L_(S0j) ofsaid limit sphere.
 2. The method of processing a source image as claimedin claim 1, wherein said decomposition space is a perceptually uniformcolor space.
 3. The method of processing a source image as claimed inclaim 1, wherein said decomposition space comprises the absoluteluminance quantity and wherein, one of the coordinates of said sourcepoint P_(S0j) then corresponding to a luminance Y_(S0j), then, todecompose said source image, the series of the n limit points P_(ljL),P_(2jL), . . . , P_(kjL), . . . , P_(njL), and the series of the ncomponent points P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj) arechosen in the same constant luminance plane corresponding to theluminance coordinate Y_(S0j) of said source point.
 4. The method ofprocessing a source image as claimed in claim 1, wherein, to decomposesaid source image, the series of the n component points P_(lj), P_(2j),. . . , P_(kj), . . . , P_(nj) are chosen so as not to belong to anyplane perpendicular to one of the reference axes of said decompositionspace.
 5. The method of processing a source image as claimed in claim 1,wherein said scrambling factor K_(S0)=K_(S0j) is common to saidplurality of decomposed pixels of said source image I_(S0j).
 6. Themethod of processing a source image as claimed in claim 1, wherein saidscrambling factor K_(S0j) is greater than or equal to 0.8.
 7. The methodof processing a source image as claimed in claim 1, wherein saidcomponent points form edges of a polyhedron, wherein said polyhedron isequilateral and centered on said source point P_(S0).
 8. A method ofprocessing a sequence of source images in which each of said images isprocessed as claimed in claim 1, wherein decompositions of said sourceimages are performed so that the succession of the series of componentpixels corresponding to the decomposed pixels of these source imagesgive rise to a fluctuation of luminance at a frequency less than theflicker limit frequency of the human eye.
 9. A method of displaying asequence of source images at a given source frequency, comprising atleast one series of component images obtained by the processing of asequence of source images as claimed in claim 8, in which, for eachsource image to be displayed, each of said corresponding componentimages is displayed in succession at a component frequency equal to ntimes said source frequency, wherein said component frequency is greaterthan the color fusion frequency of the human eye.
 10. The method ofprocessing a sequence of source images in which at least one of saidimages is processed as claimed in claim 1, wherein, if each source pixelof at least one processed source image I_(S0) which belongs to saidplurality of decomposed pixels has associated with it a motion vectorand there is defined an upper scrambling limit M_(S0q) of said sourcepixel E_(S0q) which is greater than the scrambling factor K_(S0j)associated with said source pixel and which is such that the distancesbetween the end P_(S0j) of the color source vector associated with saidsource pixel and each of the ends P_(lj), P_(2j), . . . , P_(kj), . . ., P_(nj) of the color component vectors associated with the componentpixels corresponding to said source pixel are all less than or equal toM_(S0j) times the radius L_(S0j) of the limit sphere associated withsaid source pixel, then, for each of said source pixels of saidplurality, said upper scrambling limit M_(S0j) is inversely proportionalto the modulus of the motion vector of said source pixel.
 11. The methodof processing a sequence of source images as claimed in claim 10,wherein, for each of said source pixels of said plurality, saidscrambling factor K_(S0j) associated with said source pixel is inverselyproportional to the modulus of the motion vector of this source pixel.12. A method of displaying a sequence of source images at a given sourcefrequency, comprising at least one series of component images obtainedby the processing of a sequence of source images as claimed in claim 10,in which, for each source image to be displayed, each of saidcorresponding component images is displayed in succession at a componentfrequency equal to n times said source frequency, wherein said componentfrequency is greater than the color fusion frequency of the human eye.13. A method of displaying a sequence of source images at a given sourcefrequency, comprising at least one series of component images obtainedby the processing of at least one source image I_(S0) as claimed inclaim 1, in which, for each source image to be displayed, each of saidcorresponding component images is displayed in succession at a componentfrequency equal to n times said source frequency, wherein said componentfrequency is greater than the color fusion frequency of the human eye.14. A device for displaying a source image in which each pixel isassociated with a video data triplet, comprising: a display panelcomprising a two-dimensional matrix of elementary polychrome displays;control element able to control each elementary display using a videodata triplet associated with a pixel so as to obtain the display of thispixel; a processing element able to perform a process on the sourceimage to be displayed so as to generate a series of n component imagesof said source image; wherein said control element is adapted tosuccessively display each component image at a frequency greater than acolor fusion frequency of a human eye; and wherein, in said process,said source image is decomposed in a decomposition color space into theseries of n component images and each source pixel which belongs to aplurality of decomposed pixels of this source image has a correspondingseries of component pixels respectively in each of said componentimages, wherein, if, in said decomposition space, the set of the colorvectors accessible to a display device intended to display said sourceimage forms a three-dimensional gamut of colors, and if, for each sourcepixel which belongs to said plurality of decomposed pixels, a sourcepoint P_(S0j) is used to represent the end of the color source vectorOP_(S0j) associated with said source pixel E_(S0j), and component pointsP_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj) are used to represent theends of the color component vectors OP_(1j), OP_(2j), . . . , OP_(kj), .. . , OP_(nj) associated with said component pixels of the series whichcorresponds to said source pixel, said source image is decomposed suchthat, for each source pixel of said plurality, said component pointsP_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj) associated with thecomponent pixels corresponding to said source pixel are all located insaid three-dimensional color gamut and the barycenter of said componentpoints P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj) corresponds tosaid source point P_(S0j), wherein, for each source pixel of saidplurality, if, still within said decomposition space, there is defined alimit sphere which is centered on the source point P_(S0j) associatedwith said source pixel and the surface of which contains a series of nlimit points P_(ljL), P_(2jL), . . . , P_(kjL), . . . , P_(njL), thebarycenter of which approximately coincides with the source pointP_(S0j) and which present the greatest possible distance between themwhile being included, including limits, in said three-dimensional gamut,and, if a scrambling factor K_(S0j) of said source pixel E_(S0j) of saidsource image I_(S0j) is defined which is less than or equal to 1 andgreater than or equal to 0.5, said source image is also decomposed suchthat the distances between said source point P_(S0j) and each of saidcomponent points P_(lj), P_(2j), . . . , P_(kj), . . . , P_(nj)associated with the component pixels corresponding to said source pixelare all greater than or equal to K_(S0j) times the radius L_(S0j) ofsaid limit sphere.